RFC: libVLC: use microseconds (us) for libvlc_time_t ?

Only the last commit should be reviewed.

• libvlc_clock() returned int64_t and used us.
• New player watch timer API used us.
• Lot of set/get time use libvlc_time_t as ms.
• timeout are using int as ms (not changed)

**Questions: **

• OK with the change ?
• Should we keep timeout as int / ms ? (Or use libvlc_time_t/us)

If accepted, will be merged on top of !3503 (In order to have all major libVLC changes in one shot)